The invention pertains to an apparatus for forming a single face web of corrugated paperboard. More particularly, the invention relates to a corrugating roll assembly comprising a large diameter corrugating roll (i.e. a bonding roll) and a small diameter corrugating roll in which the small diameter roll is resilient so that it is capable of deflection in the vicinity of the corrugating nip in order to cushion impact as the rolls mesh along the corrugating nip.
In the manufacture of corrugated paperboard, a single facer apparatus is used to corrugate the medium web, to apply glue to the flute tips on one face of the corrugated medium web, and to bring a liner web into contact with the glued flute tips of the medium web with the application of sufficient heat and pressure to provide an initial bond. For many years, conventional single facers have typically included a pair of fluted corrugating rolls and a pressure roll, which are aligned so that the axes of all three rolls are generally coplanar. The medium web is fed into a corrugating nip formed by the interengaging corrugating rolls. While the corrugated medium web is still on one of the corrugating rolls, adhesive is applied to the flute tips by a glue roll. The liner web is immediately thereafter brought into contact with the adhesive-coated flute tips and the composite web then passes through the nip formed by the corrugating roll and the pressure roll.
In the past, the fluted corrugating rolls have typically been generally the same size as each other. More recently, a significantly improved single facer apparatus has been developed in which the corrugating rolls comprise a large diameter bonding roll and a substantially smaller diameter roll, with the ratio of diameters about being 2.2:1 or greater. Such apparatus is disclosed in U.S. Pat. Nos. 5,628,865, 5,951,816, and 6,012,501 and application Ser. No. 09/244,904, filed Feb. 4, 1999, now abandoned, all of which disclosures are incorporated herein by reference. In accordance with these disclosures, the single facer typically includes a backing arrangement for the small diameter corrugating roll to prevent axial bending of the roll and to assure a uniform nip pressure along the full length of the interengaging flutes. One preferred backing arrangement includes a series of axially adjacent pairs of backing idler rollers, each pair having a backing pressure belt entrained therearound. Each of the pressure belts is positioned to bear directly against the fluted surface of the small diameter corrugating roll on the side of the small corrugating roll opposite the corrugating nip. Each pair of associated idler rolls and pressure belts is mounted on a linear actuator, and can thus engage the small diameter corrugating roll with a selectively adjustable force. The application of force against the small diameter corrugating roll, in turn, applies a uniform force along the corrugating nip between the small diameter roll and the large diameter roll and along the full length of the nip. Typically, a force of approximately 130 lbs. per linear inch is desirable for properly fluting a medium web at typical line speeds.
The impact of the flutes on the small diameter corrugating roll against the flutes on the large diameter corrugating roll along the corrugating nip can cause undesirable vibrations that can be detrimental to the quality of corrugation. More specifically, chordal action due to the interengagement of the roll flutes causes the small diameter roll to move up and down. The center axis of the large diameter roll is analytically stationary, and vibrational energy is transmitted primarily to the small diameter roll and to the belted backing arrangement. It has been found that excessive vibrations of the belted backing arrangements is sometimes evident under certain high-speed operating conditions, especially when the system is operated at or near the natural resonant frequency of the system. Vibration also results in increased noise and rapid wear. In severe cases, high vibration led to loss of flute caliper, cutting of the medium web and flute fracture.
In U.S. Pat. No. 6,170,549, the disclosure of which is also incorporated by reference herein, the small diameter corrugating roll is made to be resilient, e.g., constructed using an inner steel tube or carbon fiber tube having approximately a four inch outside diameter and a xe2x85x9 inch wall thickness. In the preferred embodiment, the roll is a composite roll in which the flutes are made of a sacrificial material such as reinforced phenolic resin as described in the above-identified abandoned application Ser. No. 09/244,904. The flutes are preferably cut in a resin sleeve mounted on the outside surface of the resilient steel or carbon fiber tube with epoxy. The resilient tube deflects inward as the flutes on the small diameter roll impact the flutes on the large diameter roll at the corrugating nip. Flutes made of a sacrificial phenolic resin or other similar material assist in cushioning the impact, although deflection of the resilient tube accounts for a substantial portion of the cushioning. The flutes on the small diameter corrugating roll have a different profile than the flutes on the large diameter corrugating roll such that there is a clearance between flute tips on the large diameter bonding roll and the gullets or roots of the flutes on the small diameter corrugating roll. This arrangement was intended to assure that the small diameter corrugating roll follows the bonding roll more consistently.
Extensive testing of small diameter corrugating rolls made in accordance with the teaching of U.S. Pat. No. 6,170,549 revealed that a sacrificial fluted layer exhibits unsatisfactory wear characteristics and a short wear life under certain conditions of use. Furthermore, the use of different flute profiles on the large and small diameter corrugating rolls tend to increase the amplitude of small roll deflection.
To address the foregoing problems, the fluted resin outer layer was eliminated, a somewhat larger diameter steel roll shell was adopted and the flutes were cut in the steel shell in a conventional manner. The all steel roll used in this modification is described in co-pending U.S. application Ser. No. 09/756,888, filed Jan. 9, 2001, the disclosure of which is also incorporated by reference herein. Although some improvement in wear life of the small corrugating roll was realized, there was somewhat increased wear on the flutes of the large diameter corrugating roll and, in addition, excessive amplitude of small roll deflection and consequent vibration and noise remained problems.
In accordance with the present invention, it has been discovered that with proper support of the thin walled flexible small corrugating roll, both along its length and at its opposite journaled ends, by eliminating the difference in flute profile depths between the large and small diameter corrugating rolls, and by better matching the flute profiles, the problems of excessive wear, vibration and noise can be significantly reduced. In addition, it has been found possible to utilize even thinner walled steel roll shells. This is believed to be due primarily to a reduction in the amplitude of the deflection, thereby reducing the possibility of fatigue cracking in the wall of the roll.
Thus, in a single facer having a large diameter fluted bonding roll with a flute pattern of a given axial length and an interengaging small diameter fluted corrugating roll having a flute pattern corresponding to the flute pattern of the bonding roll and supported for rotatable interengagement with the bonding roll by a series of axially spaced back-up belts to form a corrugating nip, the improvement of the present invention comprises a small diameter corrugating roll formed from a cylindrical steel shell having a minimum wall thickness after formation of the flutes, as measured from the flute gullets to the shell ID, as little as about xe2x85x9 inch. In one preferred embodiment, the steel roll shell has an initial OD of about 5.25 inches and a wall thickness of about 0.35 inch. After flute formation, the preferred minimum wall thickness is about 0.15 inch. Preferably, the small diameter corrugating roll includes a pair of cylindrical hubs that are positioned within the opposite ends of the roll shell to support the roll for rotation; and the roll shell has an axial length sufficient to position the axial inner ends of the hubs outside the bonding roll flute pattern. Each of the back-up belts is supported on a pair of rollers and includes an actuator for applying a selected support load to the small diameter corrugating roll. In general, for a nominal 5 inch diameter roll, the minimum wall thickness after flute formation is preferably in the range of about 0.15 inch to 0.23 inch.